Television receiver



May 9, 1939. G. GRUNDMANN ET AL 2,157,170

' TELEVISION RECEIVER Filed Jan. 29, 1958 3nnentor Gustave L. GrundmannHorace C. ALI/en,

dnomeg Patented May 9, 1939 UNITED STATES PATENT OFFlQE TELEVISIONRECEIVER Delaware- Application January 29, 1938, Serial No. 187,620

7 Claims.

Our invention relates to receivers for the reception of high frequencysignals and particularly to television receivers of the superheterodynetype. i

In broadcast receivers of the superheterodyne type it is the usualpractice to prevent the intermediate frequency carrier wave fromappearing in the second detector output by filtering out this signalwave. This can readily be done since the audio frequencies are farremoved in frequency from the intermediate frequency carrier wave.

In television receivers, however, the video frequencies are so close tothe intermediate frequency that a filter for removing the I. F. carrierwave would remove a large part of the video signals. Accordingly, thesecond detector is made of the push-pull type whereby the I. F. carrierwave is balanced out.

The coupling circuit between the last I. F. amplifier and the seconddetector should, of course, have a band pass characteristic similar tothat of the preceding I. F. stages. Such a characteristic is obtained byproperly coupling two tuned circuits. In the past, these tuned circuitshave been coupled capacitively by means of a coupling condenser, theamount of coupling being determined by its capacity, or they have beencoupled inductively by placing the coils in the two circuits ininductive relation. When capacity coupling is utilized, because of thesmall size coupling condenser required and because of. the variation incapacity of such condensers due to the tolerances allowed inmanufacture, it has been found difiicult to obtain in each receiver thedesired. coupling. Furthermore, capacitg'fo oupling causes theselectivity curve to trail on tlfefhig'h frequency side when themodulating frequencies cover a band which is a high percentage of thecarrier, as in the case of television.

On the other hand, ifij; is atte -pted to obtain the desired coupling bylocating-coils in inductive relation, considerable diificu1tywill beencountered in getting sufficiently tight coupling where the modulatingfrequencies cover a wide band, as in television.

Accordingly, an object of our invention is to provide in asuperheterodyne receiver an improved means for coupling an intermediatefrequency amplifier to a second detector of the push-pull type.

A further object of our invention is to provide an improved couplingcircuit between an amplifier and a push-pull detector, which circuit hasa band pass characteristic.

A further object of our invention is to provide in a television receiveran improved coupling circuit between an intermediate frequency amplifierand a second detector.

In a preferred embodiment of our invention, a pair of detector tubes,such as diodes, are connected in push-pull relation by means of aninductance coil in a well known manner. This coil and the tubecapacities form one tuned circuit. The plate circuit of the last I. F.amplifier includes a tunable inductance coil and forms the other tunedcircuit. The two tuned circuits are coupled by including a portion ofthe first-mentioned inductance in the tuned plate circuit whereby thisportion is common to both tuned circuits.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing in which Figure 1 is acircuit diagram of a portion of a television receiver embodying ourinvention, and

Figures 2 and 3 are circuit diagrams showing modifications of thecircuit shown in Fig. 1.

Referring to Fig. 1, there are shown the last I. F. amplifier l, thesecond detector 2, and the first video amplifier 3 in a televisionreceiver of the superheterodyne type. The amplifier tube l is of anysuitable type such as a pentode having the usual cathode and gridelectrodes and a plate 4.

The detector 2 is of the push-pull type and preferably comprises twodiodes 6 and I, the diode 6 having a cathode 8 and a plate 9, and thediode I having a cathode H and a plate I2.

In the specific circuit illustrated, the diodes 6 and 1 have theirplates 9 and 12 connected to opposite ends of an inductance coil l3. Thecenter point of the coil I3 is connected to ground. This center point isalso connected to the cathodes 8 and H through an output resistor M.

In accordance with our invention, the I. F. amplifier and the seconddetector are coupled in the following manner:

The plate 4 of the amplifier l is connected through a variableinductance coil l 6 and a blocking condenser l! to a point IE on the.coil l3, preferably to a point intermediate the center point and one endof the coil.

Plate voltage for the amplifier tube I may be supplied through a chokecoil [8 connected to the junction point of coil l6 and condenser H. Theplate voltage supply includes the usual filter resistor l9 and filtercondenser 2!.

Resistors 22 and 23, which may be omitted, are provided for dampingpurposes as will be explained later.

The above-described coupling circuit includes two tuned circuits. One ofthese circuits, which may be referred to as the primary circuit, may betraced from the plate 4 through coil l6 and condenser through a portionof coil lit to ground, and from ground through the output or platecapacity of the tube indicated at 24, back to the plate 4. This circuitpreferably is tuned to the desired frequency by adjusting coil Hi, thiscoil preferably being provided with a movable magnetic core such as amagnetite core.

The second tuned circuit, which may be referred to as the secondarycircuit may be traced from ground through the upper half of coil l3,through the input capacities of diodes 6 and l, which are indicated at26 and 2?, respectively, and through the lower half of coil l3 back toground.

The secondary circuit preferably is tuned to the desired frequency bymeans of a movable core of magnetic material such as magnetite.

The primary and secondary circuits are tuned approximately to the samefrequency and coupled the proper amount to provide a substantiallyflat-topped band pass selectivity curve whereby the I. F. carrier andits video side bands are supplied to the second detector. It will benoted that the only coupling between the primary and secondary circuitsis provided by that portion of coil |3 which is common to the two tunedcircuits. The correct amount of coupling may be provided very easilymerely by connecting the lead from blocking condenser M to the properpoint on coil l3. The capacity of condenser IT is immaterial so long asit is great enough to make the condenser have a negligible impedance atthe frequencies involved.

The video signals appear across the output resistor I4 and are suppliedto the video amplifier 3. It will be understood that the intermediatefrequency carrier wave does not appear across resistor M as it has beenbalanced out.

In order to obtain the desired fiat top on the selectivity curve for thetwo tuned circuits, it is usually desirable to include a dampingresistor in one of the tuned circuits. In Fig. 1 this damping isprovided by resistor 23. As will be under stood from the description ofthe circuit shown in Fig. 3, the damping may be provided by a resistoracross coil l3 in place of resistor 23, or resistors may be utilized inboth the primary circuit and the secondary circuit.

The resistor 22 is not essential but it is desirable in order to avoid ahump on one side of the selectivity curve caused by resonance in thecircuit traced from ground, through. filter condenser 2|, resistor 22,coil l8, blocking condenser II, and through the coupling portion of coill3 back to ground.

It will be noted that since the blocking condenser is connected to thecoil I3 at a point l5 away from the mid-point of the coil, the circuittends to be unbalanced a certain amount. The balance of the circuit isrestored to a large extent by locating the magnetic core which tunescoil 3 on that half of the coil which is opposite the half to whichcondenser I! is connected.

For example, as indicated by conventional. symbols in Fig. 1. if the tapI5 is on the upper half of coil l3, the movable magnetic core ispositioned in or adjacent to the lower half of coil l3. Preferably, forthe best balance, the core extends from the lower end of the coil l3about half way into the lower half of the coil.

In Fig. 2 the circuit. is similar to that in Fig. 1 except that theplate voltage i supp d to the amplifier in a different way. In the twofigures,

like parts are indicated by the same reference numerals.

In Fig. 2 an operating voltage is supplied to the plate 4 through aplate resistor 3|. While this requires a higher plate voltage than doesthe circuit in Fig. 1 because of the voltage drop in resistor 3|, it ispreferable, from one viewpoint since there are no undesirable resonanteffects to be corrected. The necessary damping for the coupled tunedcircuits is provided in the primary circuit by the plate resistor 3|.

A preferred embodiment of our invention is shown in Fig. 3, like partsin Figs. 1, 2 and 3 being indicated by the same reference numerals. Thetwo circuits are the same except for the features which will now bediscussed.

Fig. 3 shows the preferred way of supplying voltage to the plate ofamplifier I. It is supplied through a resistor 36 which is connected tothe junction point of coil Hi and condenser ll. Since this is a pointhaving lower impedance to ground than the plate 4, the resistance ofresistor 35 may be less than that of resistor 3| whereby less platevoltage may be supplied from the plate voltage supply. As in Fig. 2,there are no undesirable resonant effects.

It is preferred to provide the necessary damping for the tuned circuitsby means of a resistor 32 connected across the coil I3.

In place of the separate diode tubes 6 and a single tube 3! containingtwo sets of diode elements may be utilized. The tube 31 contains acathode 38 and cooperating plate 39, and a cathode 4| and cooperatingplate 42. The cathodes are indirectly heated, the heating circuit beingprovided with a suitable filter condenser 43.

In this particular circuit the cathodes 38 and 4|, instead of theplates, are connected to opposite ends of the coil l3 whereby the outputsignal has the desired polarity, thus avoiding the use of a polarityreversing amplifier. The plates 39 and 42 are connected together, anoutput resistor 44 being connected between the plates and ground.

The video signal appearing across resistor 44 may be supplied to thevideo amplifier through a coupling condenser 46 and a coil 41 whichresonates with the input capacity of the tube 3 at the high frequencyend of the amplifier frequency range to extend the frequency range. Adamping resistor 48 is shunted across the coil 41.

Automatic volume control may be provided by means of a connection fromthe diode plates 39 and 42 to the control grids of preceding tubesthrough a resistor 49 and a conductor 5|, a condenser 52 having suchcapacity as to give the desired time constant.

We claim as oiir i'nvention:

1. In a superheterodyne receiver, an intermediate frequency amplifierhaving a plate electrode, a push-pull second detector, said detectorhaving an input circuit which includes an inductance coil, the mid-pointof said coil being connected to ground for signal frequencies, aconnection from said plate electrode to a point on said coil other thansaid mid-point, said connection including an inductance coil and ablocking condenser in series, and means for applying an operatingvoltage to said plate electrode, the circuit including said lastmentioned coil, a portion of said first-mentioned coil and the outputcapacity of said amplifier being tuned to a certain frequency, and thecircuit including said f rst-mentioned coil and the input capacity ofsaid detector being tuned at least approximately to said certainfrequency, the coupling between said tuned circuits which is provided bysaid coil portion being such as to produce a substantially flat toppedselectivity curve.

2. The invention according to claim 1 characterized in that said meansfor applying an operating voltage to said plate electrode comprises aplate resistor which is connected between a source of plate voltage andthe junction point of said second-mentioned coil and said blockingcondenser.

3. The invention according to claim 1 characterized in that a dampingresistor is connected across said first-mentioned coil.

4. In a superheterodyne receiver, an intermediate frequency amplifierhaving an output electrode and a certain amount of output capacity, asecond detector comprising a pair of rectifying devices connected inpush-pull relation by means of an inductance coil having its mid-pointconnected to ground for signal frequencies, said detector having acertain amount of input capacity, a connection from said outputelectrode to a point on said coil removed from said mid-point, saidconnection including an inductance coil and a blocking condenser inseries, means for applying an operating potential to said outputelectrode, the circuit including said last coil, a portion of said firstcoil and said output capacity forming a tuned primary circuit, saidfirst coil and said input capacity forming a tuned secondary circuit,said tuned circuits being so tuned and coupled as to form a band passfilter.

5. In a superheterodyne receiver, an intermediate frequency amplifierhaving an output electrode, a pair of detecting devices connected inpush-pull relation to form a push-pull second detector, each of saiddevices having a pair of cooperating electrodes, the input circuit ofsaid push-pull detector including an inductance coil having one endconnected to one electrode of one of said pairs of electrodes and havingthe other end connected to one electrode of the other of said pairs ofelectrodes, the mid-point of said coil being connected through an outputresistor to the other electrodes of said pairs of electrodes, said coiland the input capacity of said detecting devices forming a tunedsecondary circuit, means including an inductance coil for connectingsaid output electrode to a point on said first coil which is on one sideof said mid-point whereby said second coil, a portion of said first coiland the output capacity of said amplifier forming a tuned primarycircuit, said primary and secondary circuits being so tuned and dampedand being so coupled through said coil portion as to form a bandpassfilter having a pass range in the region of said intermediate frequency.

6. The invention according to claim 5 characterized in that said secondinductance coil is tunable for tuning said primary circuit.

'7. The invention according to claim 5 characterized in that tuningmeans is provided for varying the inductance of said first coil wherebysaid secondary circuit may be tuned, said tuning means comprising a coreof magnetic material positioned adjacent to that end of said first coilwhich is on the opposite side of said mid-point from the point to whichsaid output electrode is connected.

GUSTAVE L. GRUNDMANN. HORACE CLIFFORD ALLEN.

